1. Technical Field of the Invention
The present invention relates to the field of electronic systems, more particularly to data storage and transmission systems.
2. Prior Arts
Multi-level memory cell can be used to improve the storage density. A multi-level cell (e.g. an N-level flash cell) can store and represent more than two states, e.g. by having N (sates-per-cell, N>2) Vt (threshold voltage) levels. In a conventional multi-level flash, b (bits-per-cell) is an integer. Accordingly, after successfully putting the flash with 2-bit cell into mass production, the industry immediately starts to develop 3-bit cell and 4-bit cell. Although migrating b from 1 to 2 might be easy (N increases from 2 to 4—a difference of 2), from 2 to 3 or even 4 proves quite difficult. This is because, after b=2, each single-step increment of b involves significant increase of N: for example, for b=3, N becomes 23 (=8), which is 4 levels more than b=2; for b=4, N becomes 24(=16), or 8 levels more than b=3. For a given total Vt window (TVW, e.g. 1.5V–6V), this significant increase of N will dramatically reduce the allowed Vt distribution width (for each Vt level) and their separation gap. For example, for b=2, the Vt distribution width can be 0.5V with a separation gap as large as 1.0V (FIG. 1A); for b=3, the Vt distribution width is more than halved to 0.2V with a separation gap of 0.4V (FIG. 1B); for b=4, the Vt distribution width becomes as small as 0.1V with a separation gap of 0.2V (FIG. 1C). To achieve these numbers, it may incur considerable research and development cost, and lost time-to-market. Accordingly, the present invention discloses a fraction-bit storage system. It abandons the conventional approach of incrementing b by 1, but allows increments of N by as little as 1 between product generations. This concept can be readily extended to other data storage and transmission systems.